Causes and Consequences of Differential Growth in Birds. A Behavioral Perspective.

Avian growth consists of the coordinated development of a number of morphological characters that may involve developmental plasticity and trade-offs. Interspecific studies show that the selection pressure from the risk of predation has driven the evolution of faster development, with ground-nesting taxa tending to have precocial offspring and cavity-nesting taxa having altricial offspring. Intraspecific studies, meanwhile, show that while nestlings preferably allocate resources to body mass and structural size increases, they also show a variety of trade-offs when experiencing adverse conditions in the nest. Between-brood studies show that in response to high levels of sibling competition and ectoparasites, nestlings allocate resources toward gape and wing development, thereby facilitating effective sibling competition and rapid fledging, respectively. Within-brood studies, meanwhile, show that, when genotypic and/or phenotypic inequalities result in asymmetric sibling competition, the smaller nestling/s generally allocate resources toward wing growth which facilitates simultaneous fledging with their larger siblings. Therefore, growth trade-offs are adaptive in the short term, but the limited evidence suggests that they are maladaptive in the long term, as allocating resources away from body mass results in lower postfledging survival.